In recent years, cellular telephones have emerged as a must-have appliance among mobile professionals and consumers alike, growing in popularity every year since they were first introduced more than fifteen years ago. Cellular telephones, once considered a status symbol by many, have become an integral link to our telecommunications infrastructure in support of an increasingly mobile society. Their widespread use for both voice and data communications has resulted from significant progress made in their portability, network services availability, and the miniaturization and declining cost of chips, memory, and other components. These factors, combined with a widespread public understanding of how mobile communications can enhance business and personal communications, as well as contribute to personal security, have resulted in the sustained high growth of cellular telephone usage.
The Advanced Mobile Phone Service (AMPS), which is the standard for analog cellular networks today, operates in the 800-900-MHz range. AMPS set the stage for the explosive growth of cellular service, which continues today. AMPS supports automatic roaming, so that mobile phone users can continue to use their phones as they move into an area served by a different network. However, analog signals have their limitations, and digital technology is advancing on all communication fronts. A digital version of AMPS solves many of these problems while providing increased capacity and a greater range of services. Both analog and digital signals operate in the 800-MHz band and can coexist with each other.
Digital radio transmission can be implemented with time division multiple access (TDMA) as the underlying technology. TDMA provides ten to fifteen times more channel capacity than AMPS networks and allows the introduction of new feature rich services such as data communications, voice mail, call waiting, call diversion, voice encryption, and calling line identification. A digital control channel supports such advanced features as a sleep mode, which increases battery life on newer cellular phones by as much as ten times over the current battery capabilities of analog phones. Digital can also be implemented with code division multiple access (CDMA) technology to increase channel capacity by as much as a factor of twenty.
The radio frequencies used for communication between the mobile user and the cell site are in the range of 825 to 890 megahertz (MHz). Separate channels are utilized for transmitting and receiving voice communications, and the telephone equipment allows transmit and receive channels to be utilized simultaneously so that the parties communicating with each other experience a full-duplex conversation not unlike that of a conventional wireline telephone. Additional radio communications between the telephone or handset and the cell site takes place over control channels that exchange data between the telephone and the cellular network as to the active phones operating within a particular service area. These control channels also provide functions critical to the establishment of calls and the management of the voice communications channels.
Since a cellular telephone is so dependent upon a radio link to establish and maintain communications, most of the factors that affect their operation are related to aspects of radio technology. Some of these factors are outside the control of the end user and are specific to the engineering of the carrier's network. The location of cell sites, proximity of adjacent cells, transmitter power, receiver sensitivity, and antenna location can all have a significant impact on the quality of communications. In many locations, service quality between providers is virtually indistinguishable. At the same time, it is commonplace that each service provider will have areas in which strengths and weaknesses exist, especially pertaining to signal coverage in any specific location. The areas of strengths and weaknesses may or may not coincide.
An additional factor somewhat beyond the control of the user is that of network traffic loading. Service can suffer even on the best of networks merely due to the congestion that results when too many users attempt to access the network at once.
In addition to classic cellular telephony, Personal Communications Services (PCS), satellite based communication, and Specialized Mobile Radio (SMR) represent later advancements in cellular telephony. By the year 2002, it is anticipated that as many as sixty million individuals in North America will subscribe to one or more of the PCS services then available. There are several categories of PCS: narrowband, wideband, and unlicensed. Narrowband PCS is intended for voice communication, two-way paging, and other types of communications that handle small bursts of data. These services have been assigned to the 900-MHz frequency range, specifically 901 to 912, 930 to 931, and 940 to 941 MHz.
Wideband PCS is intended for more sophisticated services. In addition to supporting voice communication, wideband PCS supports file transfers and LAN interconnection. These types of services will compete with cellular communications and use a frequency range that encompasses microwave users. Some microwave systems operate from 1850 to 1990 MHz. The FCC has opened up this range for wideband PCS with the understanding that microwave users will eventually migrate out of that area of the spectrum.
With the current generation of cellular service, subscribers can activate and modify their own cellular telephones without third-party involvement. Illustrative of such over-the-air programming systems are Lucent Technologies' AUTOPLEX Series II cell sites with Digital Control Channel (DCC) software based on the IS-136 standard. The system enables cellular service providers to offer enhanced features and services over existing TDMA-based digital cellular telephone networks. DCC allows cellular operators to offer multiple digital wireless services, including over-the-air programming, tailored to individual subscriber needs. It interworks with existing analog infrastructure, providing operators a gradual and cost effective migration to digital. Nortel (Northern Telecom) also supports the Digital Control Channel in its DMS-MTX wireless Systems. Multimode and multiband refers to a type of wireless system that supports more than one technology for its mode of operation and more than one frequency band. An example of a multimode wireless system is one that supports both American Mobile Phone Standard (AMPS) and Code Division Multi-Access (CDMA) systems for analog and digital communication, respectively. An example of a multiband wireless system is one that supports both 800 MHz and 1900 MHz for cellular and Personal Communications Services (PCS), respectively. It will be appreciated that wireless telephone systems can be both multimode and multiband, depending on the standards and frequencies supported.
Multimode and multiband wireless systems allow operators to expand their networks to support new services where they are needed most, expanding to full coverage at an economically sound pace. From the subscriber perspective, multimode and multiband wireless systems allow them to take advantage of new digital services that are initially deployed in large cities, while still being able to communicate in areas served by the older analog technologies.
QualComm has been offering dual-mode AMPS/CDMA handsets since 1995. Its QCP-800 portable cellular phone operates at 800 MHz. Using CDMA technology, the QCP-800 portable phone offers coverage while transmitting at RF power levels of only a fraction as much as an analog cellular phone. QualComm also now offers a dual-band, dual-mode phone that provides expanded coverage for today's PCS only subscribers. The almost universal modern day offering of telephonically available emergency assistance through a 911 type service is now regarded as a virtual necessity. Public Safety Answering Points or Positions (PSAPs) have been defined and established to serve this purpose. Pubic Safety Answering Points are customarily segmented as "primary," "secondary," and so on. The primary PSAP is the first contact a 911 caller will get. Here, the PSAP operator verifies or obtains the caller's whereabouts (called locational information), determines the nature of the emergency, and decides which emergency response team or tearms should be notified. In most cases the caller is then conferenced or transferred to a secondary PSAP from which help will be dispatched. Secondary PSAPs might be located at fire dispatch areas, municipal police force headquarters, or ambulance dispatch centers. Often the primary PSAP will answer for an entire region.
Wireless telephones have received wide acceptance for use in cellular, PCS, and like systems, and wireless user premises equipment applications. In order to satisfy the PSAP need, such telephones and the systems in which they are operated must provide a PSAP service which is at least roughly comparable to the service provided in the wired network.
When a user makes an emergency 911 call on a standard wired telephone, the location of the user is quickly determined because the physical location of the telephone is known and unchanging. It has been noted that when a user makes an emergency 911 call on the new wireless systems it may be difficult to identify the exact location of the user, thereby making it difficult to provide emergency service to the caller in a timely manner. The reason for this is that a user operating within a wireless telephone system is not bound to remain in one given physical location, since the users can travel anywhere within the physical bounds of the total system. Thus as a user moves about physically or roams, the telephone call is "handed off" from one base station to another. Thus, when operating within a wireless system it is necessary 1) for the user to always have access to the wireless telephone system within a reasonable period of time, 2) for the 911 system to identify the base station through which the call is being made, and 3) that a physical location determination of the user handset relative to the base station be easily and quickly computed. Based on the realization of these added requirements, the FCC has defined an "Enhanced 911" requirement which must provide these capabilities in all future wireless telephone systems.
Modern wireless telephone systems have largely overcome the early problem of providing satisfactory determination of mobile station location in a 911 or PSAP situation. Typical examples of such mobile locating systems are described in U.S. Pat. No. 5,388,147, issued to Grimes, Feb. 7, 1995, for Cellular Telecommunication Switching System for Providing Public Emergency Call Location Information, U.S. Pat. No. 5,548,583, issued to Bustamante, Aug. 20, 1996, for Wireless Telephone User Location Capability for Enhanced 911 Application, and U.S. Pat. No. 5,519,760, issued to Burkowski et al. May 21, 1996, for Cellular Network Based Location System. However, the satisfactory use of wireless telephone systems with PSAPs for summoning emergency aid still faces other signicicant obstacles.
The reliability and quality of mobile wireless service is notoriously dependent upon the characteristics of the link which is provided. The characteristics of the link in turn are dependent on a large number of differing types of variables. By way of illustration these variables include but are not limited the following: In a cellular system the changing position of the mobile unit within the active cell affects the interface to the system. All mobile wireless systems must cope with the frequently unpredictable nature of radio wave propagation. Obstructions and fading cause disturbances. Omnidirectional antennas are needed to reach the remote units of roaming users, and user carried units have a relatively low limit to the power that they can use and radiate. The remote unit frequently operates in an undesirable noise environment, for example, noise from auto ignition and auto charging systems. The base station is attempting to communicate with a target which may be continually moving. Changes in propagation cause frequently unpredictable variables in the quality of the link.
While the foregoing variables are illustrative of numerous potential obstacles to establishing and maintaining a quality radio link, they do not constitute the sole type of obstacle. As an example, the quality of the communication link may be fully acceptable, but the confidence level in locating the roamer may be only 2 percent. In a variation of this scenario the caller makes a fully acceptable connection to the PSAP but the PSAP attendant says "I am unable to locate you." This particular problem creates abortive circumstances independent of the quality of the available signal path. In addition, when speaking of the signal link, it is the individual user who is the ultimate subjective judge. If the user deems the connection objectionably deficient for any of a variety of subjective reasons, and hangs up to try again, the objectively acceptable link has failed. However, if the dissatisfied emergency caller redials immediately after hang-up, the network reestablishes virtually the exact same unsatisfactory link from the mobile station to the PSAP.
A need exists for overcoming these problems in mobile wireless telephone operation relying on PSAPs.